1. Field of the Invention
The present invention relates to DC battery backup systems and, in particular, to monitoring such systems to predict battery health and functionality.
2. Description of the Related Art
Many facilities, sites, or types of equipment are normally powered from the external AC commercial power grid. Often, such sites use batteries as an emergency backup power supply to provide power to equipment or other devices at the site in case of an AC power supply failure. The backup batteries and associated equipment are sometimes referred to as a DC power plant or battery plant.
Each such battery plant typically includes one or more battery strings, often coupled in parallel. Each battery string typically includes a number of series-connected rechargeable cells. The general term xe2x80x9cbatteryxe2x80x9d may also be used to refer to batteries such as battery strings. The AC power typically is converted to DC with a rectifier. The DC power is used to keep the battery string charged (known as a float-charge operation) and also to power various equipment such as telephone switching equipment. The battery terminals are thus directly coupled in parallel with the output of the rectifier and with the input terminals to the equipment requiring DC power.
Thus, so long as AC power is provided to the rectifier, it provides DC power to power the equipment and also to keep the battery strings continually charged (called a float charge operation or condition), or to recharge them after a discharge. If AC power is lost, the DC battery strings immediately begin discharging and power the equipment. Such battery backup systems are sometimes referred to as xe2x80x9cuninterruptible power suppliesxe2x80x9d because the battery system immediately supplies the necessary energy upon loss of AC power. These systems are usually designed so that the load is never aware of the AC power failure, as long the backup DC power is provided as expected by the battery plant.
In order to provide services to rural areas, telecommunications equipment is often deployed in small facilities such as cabinets and controlled environmental enclosures, sometimes referred to as outside plant (OP) power system sites. These sites are typically unmanned and remote from the central office (CO). Lead acid xe2x80x9cwetxe2x80x9d cell batteries are often used in COs but valve-regulated lead acid or xe2x80x9cdryxe2x80x9d batteries have become the dominant type of battery used at remote OPs, because they are sealed (no water) and have lower maintenance. Such dry batteries also have less power and are typically more expensive than more maintenance-requiring wet cells. A 24-cell dry cell battery typically provides 48V.
A new dry cell battery is typically designed to last 10-20 years, but may only last 2-3 years if a problem (e.g. corrosion or short-circuited cell) develops. This may be due to misuse, too-high temperatures, etc. In addition to prematurely short life of such batteries, dry cell batteries and other batteries often experience unexpected failures and general capacity loss, and thus require many unplanned and costly service calls.
In many applications, it is critical that the batteries of the power plant be functional and ready to supply sufficient backup power if AC power is lost. For example, a battery plant may be designed and expected to provide an 8-hour discharge. If, however, the battery has an unknown problem and only lasts one hour when AC power goes out, telephone service may drop out after one hour and may not be restored for the several hours it takes power to be restored. Or, if a battery string is supposed to last a given time (e.g. 20 years) and will normally be replaced at some battery age close to but before the expiration time (e.g., at 19 years), a prematurely aged battery may not be replaced in time (e.g., a battery may have an effective or xe2x80x9ctruexe2x80x9d age of 20 years after only 5 years of service due to various factors such as prolonged high operating temperatures).
Thus, it is important to monitor battery strings of a given battery plant to predict battery failure and other battery performance or status parameters or measures related to battery health or performance, so that action can be taken ahead of time to ensure that the problem is addressed, i.e. to ensure that the battery strings will perform as expected upon loss of AC power and to ensure that the batteries are adequately maintained, inspected, replaced, and the like.
Some causes of impending failure are relatively easy to detect, such as grid corrosion, short circuits in cells, post leaks, excess buildup on the negative plates, and the like. However, some of these causes may only be detected if a complete inspection is made as part of a site visit. Many types of battery monitoring equipment and techniques can predict battery health only with on-site manual application and testing. Such routine site visits can be cost-prohibitive and impractical.
Load discharge tests can be used, for example, to test a battery system""s ability to perform (health). However, such discharge tests reduce the remaining service life of a battery and may cause deterioration which can impair the battery""s ability subsequently.
Human personnel may also take periodic electrolyte specific gravity measurements of wet cells and may conduct visual and other checks to determine battery status. However, in systems employing sealed or dry cells, the measurement of specific gravity is not practical. Moreover, such readings are not completely reliable indicators of a battery string""s functionality.
The measurement of the open circuit voltage may also be employed, but requires disconnection from the system for 24 hours and has other disadvantages.
xe2x80x9cIntrusivexe2x80x9d tests such as conductive and impedance testing has also been used to try to predict battery failure. The required equipment is expensive, however, and must be used at the site to measure each battery string on a cell-by-cell basis. Thus, such battery testing techniques are intrusive and require a labor-intensive site visit, which increases expense and also means that there cannot be continuous testing or monitoring of the battery strings. In addition, the results of such tests are not 100% accurate.
Some battery monitors integrate directly into the power plant and continuously and automatically monitor a battery system without manual action. In order to be accurate, such equipment must be very advanced, intrusive, and expensive. Alternatively, if simpler and cheaper testing techniques are utilized, the results are much less accurate.
There is a need, therefore, for a low-maintenance, low-cost, reliable and accurate battery monitoring system for unmanned, remote monitoring of battery plants for prediction of battery health, i.e. to predict whether or not the battery strings are capable or incapable of providing uninterrupted service for a predetermined period of time if the main AC power source fails. An alarm message can then be transmitted to the CO to allow the problem to be addressed, or other action, such as automatic disconnect, may be taken.
A system for monitoring performance of one or more batteries in a battery plant facility. A set of sensors for each battery are used to measure a plurality of battery operation parameters comprising the battery current, battery voltage, and internal battery temperature of each said battery. A processor generates, for each battery, a plurality of battery status parameters comprising a true age parameter and a float capacity parameter. During a float period, the true age parameter is updated based on elapsed time corrected for battery temperature history and the float capacity is updated based on the most recent true age parameter. During a discharge period, the float capacity parameter is updated based on the evolution of plant voltage during the discharge. After the discharge period, at the beginning of a subsequent float period, the true age parameter is updated based on the float capacity parameter determined during the immediately previous discharge period.